1. Field of the Invention
The present invention relates to a scan beam light quantity distribution measurement method and a measurement apparatus of scan optical system, and more particularly to a scan beam light quantity distribution measurement method, a measurement apparatus, a measurement evaluation apparatus. The present invention also relates to an image formation apparatus with the measurement evaluation apparatus capable of evaluating quality of an optical element used in a scanning optical unit to include detecting the influence on optical performance by profile irregularities, a surface defect, an internal defect while measuring light quantity distribution of a scan beam in the scanning optical unit.
2. Description of the Prior Art
A write scanning optical unit used for an image formation apparatus such as a laser printer, a copy machine, a facsimile apparatus and so forth has a scan optical system constituted mainly by a laser light source, a collimator lens, various kinds of lenses and mirrors, a polygon mirror and so forth. In general, the image formation apparatus transforms laser beam generated from a laser light source into parallel light using the Collimator lens to irradiate to the polygon mirror, where the parallel light is deflected by a revolution of the polygon mirror. The reflected light beam is focused as a point image on a photosensitive body by an imaging lens and a mirror system. An electrostatic latent image is formed on the photosensitive drum by performing laser beam scan in the horizontal scan direction obtained by revolution of such a polygon mirror and in the vertical scan direction obtained by revolution of the photosensitive drum.
Further, it is known that toner is applied to the surface of the photosensitive drum having the electrostatic latent image thereon, thereby forming a toner image on the surface of the photosensitive drum. Subsequently, the toner image on this photosensitive drum is transferred onto a transfer sheet and fixed thereon. Thus an image is formed on this transfer sheet.
Now, when the optical elements constituting the scan optical system have abnormalities such as profile irregularities (undulation), a surface defect, an internal defect, and so forth, the scanning position of the scan beam on the photosensitive body is deviated in the horizontal scan direction, and out-of-focus condition in the depth direction of the scan beam occurs. Such position deviation deteriorates the peak light quantity of the scan beam and/or affects size of diameter and shape of the scan beam, thus causing faulty image formation.
The conventional approach for evaluating beam diameters of light beams include using a pin hole or slit in a position corresponding to surface of the photosensitive drum and setting a photo-detector immediately at the rear thereof, so that the beam diameter is measured in a stationary state. Then an evaluation is made whether the scan optical system has a defect by judging whether a point of abrupt change is present in the beam diameter thus measured.
However, this conventional measurement method of the beam diameter requires excessive measuring times when measurement across the entire scanning area in every predetermined pitch (for instance, when measuring across the entire scanning area in every 1 mm). Further, since the measurement is executed only against a stationary beam, there is problem that scanning beam characteristic cannot be measured.
Furthermore, the beam diameter is measured with respect to peak intensity of the beam and accordingly, there is the problem that the conventional evaluation does not account for absolute amount of the intensity.
Various kinds of methods have been suggested for measuring scan beams generated by an optical system. For instance, it is described in Japanese Patent Application Laid-Open No. HEI 9-43527. This application discloses a scan beam measurement method and a measurement evaluation apparatus using three pieces of beam position sensors, one beam diameter sensor, and one light quantity sensor equipped on a movable stage. In the apparatus, the beam diameter and the light quantity of the scan beam measured while moving the beam diameter sensor and the light quantity sensor with the movable stage while ascertaining the photo-sensed position of the beam position sensor.
However, in such a conventional measurement method, there are following problems, namely:
While measurements of the beam diameter are made using the beam diameter sensor and measurements of irregularity of light quantity are made using the light quantity sensor, measurements of light quantity distribution of the scan beam cannot be measured.
Because measurements of the beam diameter are performed at distant positions, this measurement method cannot measure relative to neighboring beam diameters about degree of one pixel apart due to the arrangement of the beam position sensors.
While measurements of the beam diameter and the light quantity are made while moving the movable stage, this method measures the beam diameter at only predetermined positions, and it cannot measure the beam diameter at arbitrary positions along the entire horizontal scan direction. Moreover, measurement of the light quantity are executed only for the entire luminous flux of incident light, not for the distribution of the light quantity of the beam.
While beam position is measured using the beam position sensor at three positions, evaluation of magnification error cannot be preformed because accurate distances between respective beam position sensors are not known and it is impossible to detect distance between two points of beam.
Measuring at only three positions cannot determine accurate scanning line bend-amounts, because there is restriction in the number of arranged beam position sensors.
Also, while a scanning side beam detecting sensor is provided in the scan optical system, and three pieces of stage side beam detecting sensors are provided for detecting timing of incident scan beam at the movable stage to evaluate scanning side beam detecting sensor positioning, the conventional apparatus cannot emit beam of arbitrary light emission pattern to the stage side beam detecting sensor and also does not enable the scan beam to emit a light repeatedly because of the predetermined position of the stage side beam detecting sensor.
As stated previously, in conventional measurement beam diameter methods, when measuring across the entire scanning area (for instance, in every 1 mm) the measurements require excessive measuring times. Further, since the measurement is executed against a stationary beam, beam scanning characteristic cannot be measured.
As to a method for measuring the scan beam in the scan optical system, it can be considered to constitute a method by performing position measurement by the position detector of the movement means and synthesizing dot positions at this position, thereby detecting the dot position in the entire scan system.
In such a measurement apparatus, since measurement time and oscillation and so forth should be considered, instead of repeating movement and stopping for every measurement, beam position measurement is usually performed while moving two-dimensional sensor in the horizontal scan direction. However, when positions are stored at the time of generation of scanning reference signal and the beam positions are detected in the entire scanning area, measurement time errors described later are generated. This error cannot be ignored with respect to position instrumentation error when positioning instrumentation with high accuracy requirements.
Speed fluctuation of movement mechanism and influence of observation time upon position accuracy are now explained. Speed of a movement mechanism is taken to be V (mm/sec), movement speed fluctuation is taken to be xcex4v (mm/sec), and scanning frequency of a laser line is taken to be F (1/sec). In scanning the line, movement of the movement mechanism L (mm) is xe2x80x9cL=(V+xcex4v)/F,xe2x80x9d movement amount error xcex4L when speed fluctuation of the movable stage is about degree of xc2x110% is xe2x80x9cxcex4L=xc2x10.1V/F.xe2x80x9d In the case of V=100, F=2000, xcex4L becomes xe2x80x9cxcex4L=xc2x15xc3x9710Exe2x88x923 (mm)=xc2x15 xcexcmxe2x80x9d.
About correspondence to this deviation from the reference position, one can mount a triggering photo-detector PD on a front stage of a conventional detection system. However, by adding the triggering PD, not only does the apparatus configuration become complicated, but also requires extremely complicated lighting control such that the scan beam enters the triggering PD.
Further, if there is fluctuation in between a position detecting synchronizing signal and the time that position detection is actually executed, the beam position can be incorrect. For example, a fluctuation of detection time of PVHF under the aforementioned settings will result in a position error of 10 xcexcm. Since the beam diameter of the scan system is smaller than 100 xcexcm, an error of 10 xcexcm is significant.
Furthermore, in order to minimize fluctuation of this detection time, it is necessary to constitute systems by using hardware dedicated for detection or by using real time OS whose real time characteristic under order of xcexcsec is guaranteed. Accordingly apparatus cost rises in comparison with the case that control is executed with general-purpose PC.
In view of the foregoing, it is one object of the present invention to provide a method and apparatus for measuring a scan beam light quantity distribution in a scan optical system, taking into account profile irregularities (undulation), surface defects, internal defects, and so forth while retaining high accuracy.